1. Field of the Invention
The present invention relates to an apparatus for diagnosing the residual charge of a battery, for example, one mounted in a vehicle such as an automobile.
2. Description of the Related Art
In the conventional techniques for determining the residual charge of a battery mounted in a vehicle, the terminal voltage, the internal resistance, or the specific gravity of the electrolyte of the battery is measured, and the measured value of one of these factors is utilized to determine the residual charge of the battery. Such conventional residual charge determining techniques are disclosed in, for example, the specification of Japanese Patent Laid-Open No. 55-58739.
However, since the internal resistance of the battery and the specific gravity of the electrolyte change with the passage of time, the results obtained utilizing either of these factors may vary due to the time during which the battery has been used or the condition in which that battery is used.
The residual charge of a battery may be determined relatively easily and accurately by measuring the terminal voltage of the battery. However, the terminal voltage of a battery may be accurately correlated with the residual charge only in states where the battery has not been affected by charging or discharging and where a sufficient time has elapsed since the last charging or discharging so that the effect of the chemical action of the electrolyte caused by the charging or discharging of the battery can be ignored.
Thus, the terminal voltage of the battery must be measured only after the lapse of a certain time from the time when the operation of the engine has stopped and the battery has been disconnected from the load. However, it is a troublesome task for a driver to measure the passage of a certain time, measure the battery terminal voltage after a certain time has elapsed, and determine the residual charge of the battery utilizing the measured value.
Further, if the terminal voltage of the battery is measured immediately before the engine is restarted, it may be impossible to obtain an accurate determination result of the residual charge of the battery because the time the engine has been stopped may vary and the battery may be still under the influence of the chemical action of the electrolyte.